About a month ago i setup my 55g with around 50lbs of live rock filter/everything that is needed. (only missing the skimmer wich is next on my list)

Now i have 2 damsel since a week ago and they get along pretty well togheter minus the chasing around at times. Yesterday i bought a sailfin blenny to do some clean up in there but he's getting attack by the 2 damsel (three stripes) as soon as he gets out from under the rocks. They just ram into that poor little guy

It's the nature of the damsel. I learned that the hard way. I lost about $200 worth of fish at the hand of a $5 damsel. They are aggressive and will eventually kill your other fish. At least that has been my experience. Others might disagree but I would hate for you lose fish like I did.

Damsels can be pretty mean, but a lot of it also stems from the order the fish go in the tank. Normally, you want the most timid fish going in first, and finishing with the most aggressive fish. That way, the timid fish get some time to stake out territory and consider the tank their own.

Unfortunately, seems like a lot of people start with damsels - either because they're inexpensive, or because they cycled with them. And since they were first, they're just defending "their" territory from the newcomers.

So what you're seeing is normal, but I wouldn't say "at first". Doubt the aggression will go away.

I got rid of my damsel all together. It would leave the others alone during the day and save all the aggression for the night when no one was looking. It started out little pieces missing from their fins. So check the fish that it's chasing and see if any of the fins look like they had a bite taken out of them. If so, then your damsel needs to go. It will kill it.

I would suggest just getting rid of both of them. Damsels can get really nasty (not all of them but most) and will only get worse as they get bigger. You will most likely continue to see aggression from them when ever you add a new fish that is not bigger and meaner then them.